Array.prototype._map = function(fn, ctx) {
    let tmp = []
    console.log(this)
    if(typeof fn === 'function') {
        this.forEach(i => {
            tmp.push(fn.call(ctx, i))
        })
    } else {
        console.log("error! not a array")
    }
    return tmp
}

console.log([1,2,4,6]._map(i => 2 * i))